Solar and wind power provide carbon-free electricity. But their generation is tied to the vagaries of nature. That is why, to achieve a zero-carbon electric grid, technologies that can store that energy for when it’s needed will have to play a big role.

While lithium-ion batteries have started to meet some of the need for storage, the metals needed to make them are not plentiful enough for large-scale energy hoarding. So entrepreneurs around the world have been looking for alternatives.

At Quartz, we’ve written about companies working on reversible sulfur power-plants, injecting water underground, running “refrigerators on steroids,” and using stacked concrete blocks—all with the purpose of storing excess renewable energy. Add to that list Hydrostor, a Canadian startup that’s storing energy by injecting compressed air into deep underground caverns.